Hardness compounds such as barium, calcium, magnesium, iron, silica, carbonate and bi-carbonate, fluoride and sulfate are commonly found in surface water supplies such as lakes and rivers as well as groundwater supplies such as water wells and aquifers and in aqueous industrial effluents and landfill leachates. Water containing hardness compounds is frequently purified by using water softeners and demineralizers in the form of “ion exchange resins, IX”, chemical softeners using the cold lime or hot lime softening process, reverse osmosis (RO) membranes, nanofiltration (NF) membranes and/or distillation. Industry needs purified water containing low to very low concentrations of hardness compounds and of soluble inorganic compounds in order to supply their cooling towers, low-pressure and high pressure boilers, heat exchangers and various process uses. On the other hand, the pharmaceutical and electronics' industries as well as hospitals and laboratories require high purity water that is almost completely free from inorganic compounds.
The conventional water treatment processes listed above are not suitable or efficient because, in the case of IX, the process involves the inefficient transfer of soluble and “sparingly soluble” water impurities to a resin bed which must be regenerated using chemicals and/or disposed of at high cost. In the case of lime softening, large quantities of chemicals are added and large chemical waste volumes are generated. If conventional RO or NF membranes are used, substantial volumes of RO or NF membrane concentrates will be generated because the permeate recovery (percentage) from these processes is normally limited to approximately 70%-75% and the concentrates must therefore be treated further or disposed of at a large cost. Finally, the very high capital and/or operating costs associated with the direct application of distillation processes normally preclude the use of distillation as a single-step treatment method.
The main reason why high purified water recoveries >90% from “conventional” RO and NF membranes are not possible is the tendency of inorganic scale such as calcium carbonate, calcium sulfate and silica to form on the surface of the membranes as the concentration of these compounds is increased beyond their saturation values. Deposition of such compounds frequently results in the loss of purified water production (also known as loss of permeate flux through the membrane) and the eventual need for costly replacement of the membranes.
The use of chemical additives in the water supply such as acids to reduce the pH and inorganic or organic scale inhibitor compounds is practiced in the water treatment and membrane industry in order to provide some improvement in the water recovery and prevent scale formation. However, such improvement is of limited extent since no scale inhibitor is effective for all the contaminants nor for all permeate recovery ranges and therefore they do not represent a viable option for the treatment of the entire water stream.
Ion exchange resins, including strong acid cation exchange resins (SAC), weak acid cation exchange resins (WAC) or chelating ion exchange resins are used to “soften” the Influent Water introduced into RO or NF membranes by removing divalent and multivalent hardness ions, thus reducing the scaling potential of the water and enabling higher permeate recoveries to be achieved. However, this membrane pretreatment is deemed to be very costly, since it removes all of the hardness from the entire Influent Water even though the RO membranes can tolerate some hardness, especially if low-dose, effective anti-scalant chemicals are used. Leakage of hardness ions through the IX water softeners used to pretreat the Influent Water will limit the permeate recovery of the RO or NF membranes due to build-up of these hardness ions concentrations in the membrane concentrate stream, thus potentially causing premature precipitation and scale formation. The SAC or WAC cation exchange membranes used in water softening are also unable to remove weakly ionized soluble silica from the Influent Water, which can therefore limit the permeate recovery as the silica concentration increases by an order of magnitude or higher across the RO or NF membranes. Furthermore, the IX resin will become saturated with divalent and multivalent hardness ions including calcium and magnesium which must then be removed periodically by using low impurity salt solution containing up to 10% of pure sodium chloride to ensure complete hardness removal from the resin and absence of hardness leakage in the service mode.
Attempts have been made in recent years to address the hardness and silica limitations by efficiently removing the hardness from the RO or NF membrane concentrate using ion exchange, chemical precipitation or other efficient water softening methods followed by recycling of the hardness-free membrane concentrate to the influent or low pressure side of the membranes to achieve further water purification and realize permeate recoveries >95%. When chemical precipitation softening methods are used, the process complexity and capital and operating costs increase, rendering these processes less attractive. Furthermore, when ion exchange is used for softening of the membrane concentrate, the IX resin must be periodically regenerated with pure, high-strength sodium chloride salt solutions that must be prepared using purified membrane permeate. Use of these salt solutions adds to the operating cost, introduces an undesirable chemical as a new waste into the environment and adversely impacts on the overall purified water recovery of the process.
In addition to the above limitations, it is often unclear when designing a high-recovery process what the optimum water purification process would be, i.e. is it a simple reverse osmosis (RO) system? Is it an RO system that incorporates the addition of anti-scalant to increase the solubility of sparingly soluble hardness and silica compounds thus increasing the recovery, say from 60-70% to 80%? Is it an RO process that is preceded by a SAC or WAC IX pretreatment step that increases the nominal purified water recovery to 85%-90%, or is it a single stage or 2-stage high-recovery process that post-softens the RO or NF membrane concentrate using SAC IX and recycles the softened RO concentrate to achieve purified water recoveries >90% and even >95%? And will these processes need to use the undesirable sodium chloride salt solution to regenerate the IX resin.